The invention relates to a luminescent screen comprising a bearer provided with a luminescent material and to a low pressure mercury discharge lamp provided with such a luminescent screen.
In general a luminescent screen is used to convert excitation energy into radiation within a certain range of wavelengths. The excitation energy can for instance consist of an electron beam, X-rays or UV radiation of relatively short wavelengths. Among many other applications luminescent screens find application in lamps, particularly in low pressure mercury discharge lamps or fluorescent lamps. The excitation energy then consists of UV-radiation of approximately 254 nm generated by mercury present in the plasma of the fluorescent lamp. The composition of the luminescent screen is chosen in dependence on the desired spectral composition of the light radiated by the fluorescent lamp. In many applications it is desirable that the fluorescent lamp radiates only light in the visible part of the spectrum. However, in some applications for instance in lamps for influencing photochemical processes, such as polymerization, lacquer hardening, drying, curing, medical irradiation purposes or suntanning, it is desirable that at least part of the light radiated by the lamp is UV radiation, more in particular so called UV-A radiation having wavelengths between 315 nm and 400 nm. A problem associated with such fluorescent lamps is that known luminescent materials having an emission maximum in the range between 315 nm and 400 nm have a relatively low quantum efficiency.